System elements for measuring pressure in extracorporeal circuits

ABSTRACT

The aim of the invention in particular is to reduce the risk of haemolysis in extracorporeal blood circulation. To this end, a system element for releasably sealingly connecting a transducer to a fluid system comprises a measuring chamber ( 7 ) which can be connected to the fluid system in such a way as to allow throughflow. Said measuring chamber ( 7 ) is formed in a housing ( 10 ). Part of the wall of the measuring chamber ( 7 ) is formed by a membrane ( 11 ) which is considerably more flexible than the rest of the wall ( 15 ) of said measuring chamber ( 7 ). The membrane ( 11 ) has a peripheral built-up section ( 20 ) which is located on the side of the membrane ( 11 ) that faces towards the measuring chamber ( 7 ). Said built-up section ( 20 ) engages in a channel ( 19 ) which is formed in the housing ( 10 ) and which extends around the measuring chamber ( 7 ). The wall (inner wall  16 ) of the channel ( 19 ) that faces towards the measuring chamber ( 7 ) is lower than the wall (outer wall  17 ) of the channel ( 19 ) that faces away from the measuring chamber ( 7 ) ( 19 ). The invention also provides for a system element for releasably sealingly connecting a transducer to a fluid system, containing at least one sensor for converting pressures and pressure changes into electrical signals. The sensor is located in a housing ( 26 ) and at least part of the housing ( 26 ) is filled with a fluid or gel which transmits pressure. The housing ( 26 ) is closed by a transducer membrane ( 23 ) on one side. In the area ( 24 ) surrounding the transducer membrane ( 23 ), the housing ( 26 ) is configured for applying one of the system elements ( 1 ) according to one of the preceding claims. This surrounding area ( 24 ) essentially encompasses the transducer membrane ( 23 ) in a ring shape. The measuring range of the sensor has differential pressures of at least −250 mm Hg to +1000 mm Hg in relation to the surroundings. The invention also relates to a set comprising the inventive system elements.

This application is a 371 of PCT/EP01/07879 Jun. 9, 2001.

The invention relates to a system element for the releasable sealedconnection of a transducer to a fluid system, with a measuring chamberwhich can be connected to the fluid system in such a way as to allowthroughflow, the measuring chamber being formed in a housing and part ofthe wall of the measuring chamber being formed by a membrane which isconsiderably more flexible than the rest of the wall of the measuringchamber, and also a system element for the releasable sealed connectionof a transducer to a fluid system, containing at least one sensor forconverting pressures and pressure changes into electrical signals, thesensor being arranged in a housing and at least part of the housingbeing filled with a fluid or gel which transmits pressure, and thehousing being closed by a transducer membrane on one side, the housingbeing further configured in the area surrounding the transducer membranefor applying one of the system elements as claimed in claim 1, thesurrounding area essentially encompassing the transducer membrane in aring shape, and a set comprising such system elements.

Such connecting elements are known in medical technology by thecolloquial term “dome” or “pressure dome”, which originates from thedome-shaped design of the measuring chamber, or by the term“transducer”, which is understood as meaning a measuring converter in asuitable housing which converts the pressures and pressure changesusually transmitted via the membrane of the pressure dome into anelectrical signal. They serve the purpose of permitting the measurementof pressures in fluids during the examination and treatment of peopleand animals, preferably by means of electronic diagnostic and monitoringequipment.

DESCRIPTION OF THE PRIOR ART

For pressure monitoring during the flushing of body cavities, in DE 4219 888 A1, for example, there is described a flow pressure transducerwith such a connecting element, which is designed for a large volumethroughput in accordance with the intended area of use. At the sametime, the flow pressure transducer is to be designed as apressure-relief valve which bypasses a peristaltic pump when thedifferential pressure predetermined by the structural design of the flowpressure transducer on the pressure side of said peristaltic pump withrespect to the suction side of the pump is exceeded.

For monitoring the hemodynamic parameters of a patient, in particularintensive-care patients, it is customary nowadays in addition to therecording of an ECG also to record the invasive pressures into the [sic]patient monitoring, that is to say keeping a check on the state of thevital bodily functions of the patient. Depending on the degree ofmonitoring, between one and four pressures (arterial,pulmonary-arterial, LAP and venous) are measured.

For this purpose, a catheter with an integrated monitoring set is used.The positioning of the end opening of the catheter defines the measuringpoint in the patient's body. A monitoring set refers to a compilation ofthose parts which establish the connections between the patient and theso-called monitor and, usually for reasons of hygiene, are intended foronce-only use. A monitor refers to the electronic monitoring andrecording system with which the corresponding measured data areevaluated and displayed, and which if need be emits corresponding alarmsignals if measured data leave prescribed set ranges.

A general description of this, relating to the example of examination bya flow-directed catheter, is found in Buchwalsky, Rainer:Einschwemmkatheter: Technik, Auswertung u. prakt. Konsequenzen[flow-directed catheters: technology, evaluation and practicalconsequences] (Beiträge zur Kardiologie [articles on cardiology], Vol.29); Erlangen: perimed Fachbuch-Verlagsgesellschaft, 1985, pages106-109.

The monitoring set to be fastened to the catheter comprises anunventilated infusion apparatus for feeding infusion solutions to thepatient, a flushing system, which ensures a continuous flushing rate ofcustomary 3 ml/h at the catheter tip to avoid occlusion being caused bythrombi, if appropriate with a quick flushing function for specialcases, and a pressure dome. The pressure dome transmits the pressuresignal via its flexible membrane to a reusable transducer (pressuresensor). Such a pressure dome has in the past been fastened on such atransducer by a screw or bayonet connection (see in this respect DE 4219 888 A1, column 3, lines 28 to 30), or by means of snap hooks (cf. WO99/37983).

Examples of such pressure domes are to be found on an information sheet“Disposable Transducer Domes” of the company SMP Specialty MedicalProducts, Dallas, Tex., US, with reference to the models 078 to 082. Atypical transducer (pressure sensor) is described, for example, in aleaflet of the company SensoNor a.s, Horten, NO, Edition 1/95, on theproduct SensoNor 840.

Further elements of a monitoring set are the pressure hoses (marked incolor) and possibly a three-way cock, to allow medicaments to be fed in,or a blood removal system for taking blood for further investigations.

A special problem is that of venting the parts of the monitoring set inconnection with the blood system. The problems involved in venting suchsystems are generally known to the users. During the filling of thesystem (usually with physiological saline solution), air bubbles becometrapped particularly easily in the dome, i.e. in its dome-like measuringchamber above the membrane. On account of the great elasticity inherentin gases, by contrast with the virtually incompressible fluids, the airbubbles trapped there represent a barrier in the transmission ofpressure frequencies of more than a few Hertz. This has the effect ofsignificantly falsifying the transmission of the change in pressure tothe membrane, and consequently to the transducer lying thereunder, andas a result the representation of the pressure curves on the monitor.

A connecting element of the type mentioned at the beginning is knownfrom prior public use by SMP Specialty Medical Products, Dallas, Tex.,US, under the type designation 081. This “dome” is intended to fit theHewlett Packard 1290 Quartz transducer and allow itself to be fastenedon the latter by means of a bayonet connection. Lockable Luer-lockconnections with a loose threaded part, or with an external full thread,as are specified for example in DIN 13 090 Part 2, serve for connectingthe inlet channel and outlet channel to the hoses of a monitoring set.

The known connecting element is produced from a crystal-clear plastic.The measuring chamber of this connecting element is very large and has,in particular, a large diameter of approximately 23 mm. In this case,the ceiling of the measuring chamber is at the same time the upper sideof the housing. This ceiling and upper side of the housing is formed ina plane-convex manner as a magnifying lens. This is intended to achievethe effect that even small bubbles in the measuring chamber are detectedas reliably as possible by the medical care personnel.

Disposable transducers which contain the pressure-measuring sensor in aflow housing are therefore designed in the form of a simple tube in theflow chamber in order to avoid this very trapping of air bubbles.However, they have the disadvantage that the valuable electronics areintegrated in the disposable article and therefore are thrown away eachtime the monitoring set is changed and have to be disposed of along withit. To comply with hygiene requirements, such an exchange must takeplace at the latest every second day. This entails not only thedisadvantage that the still serviceable electronics are replaced withevery change, accompanied by corresponding costs, but also that thepresence of electronic components requires additional special, andconsequently cost-intensive, treatment as electronic scrap duringdisposal.

For this reason, dome systems which can be used repeatedly are becomingmore popular, at least in Europe. The valuable electronics, inparticular the pressure sensor, are located in a special housing. Such apart is usually referred to as the transducer. One or more transducersare integrated in a special retaining plate. The retaining plate isfastened, for example on an infusion stand, by means of a clamping orscrewing device. The measured pressure data are transmitted from thetransducers in the retaining plate to the monitor via one or morecables.

This problem is also not solved by a connecting element of the typementioned at the beginning, such as that described in DE 35 25 536 A1.To avoid damage, in particular to the membrane of the connectingelement, a fastening in which it is not necessary for the connectingelement and transducer to be turned in relation to one another isproposed there. For this purpose, additional fastening elements, such aspivotably mounted clamps or closing hooks elements, are to be providedat two points on the circumference of the housing of the connectingelement.

It is proposed there to allow these fastening elements or continuationsto protrude downward beyond the membrane for the handling of thefastening elements. This is intended to make it possible to checkvisually that locking of the fastening element provided has also beencarried out. Furthermore, this is intended to simplify handling when theconnecting element is removed from the transducer. However, itpractically rules out a combination with transducers fastened inretaining plates.

Furthermore, according to the teaching of this printed publication, thehousings of the pressure dome and transducer are intended to touch oneanother directly (see column 9/10 therein), whereby the membranes of thepressure dome and transducer are indeed intended to lie “snugly on eachother”, but prestressing of the membranes is considered disadvantageousand is to be avoided (loc. cit., column 4, lines 21 et seq.).

EP 0 701 830 A1 describes a device and a method preferably for use inthe area of pressure measurement in blood-carrying lines for dialysisequipment, for hemofiltration and hemodiafiltration. For this purpose,the pressure is measured indirectly via a gas column (air), which is inconnection with the liquid column in a line via an elastic membranelocated in a housing. The pressure of the gas column is registered bymeans of suitable and customary sensors (pressure pickups). To extendthe restriction of the measuring range caused by the mechanicallylimited deflectability of the membrane and the compressibility of thegas column, there is proposed a device and a method with which theamount of gas in the gas-carrying part of the measuring instrument isincreased or decreased in dependence on the gas pressure to be measuredand with the result of maintaining mobility of the membrane andconsequently the relaying of changes in pressure.

Changing the amount of gas takes place by means of pumping devices inthe form of a peristaltic pump actuated by means of a control system.The compressibility of the volume of gas to be interposed according tothe EP application has the effect of forming a kind of acoustic low-passfilter, which damps or suppresses rapid changes in pressure orrelatively high-frequency pressure oscillations. The system according tothe printed publication is suitable in practice only for measuringstatic pressures or for monitoring mean values, it being possible to setthe formation of the mean pressure value by the damping characteristicsof the system, for example by means of the volume of gas switchedbetween the membrane and pressure transducer. For arrangements of such atype, an upper cut-off frequency of approximately 0.1 Hz is known fromprior use for registering changes in pressure.

The publication also describes in detail a method with modifications asto how the desired setting of the position of the membrane, andconsequently of the measuring range, can take place without the positionof the membrane having to be registered mechanically or observed byusing an electronic control system through successive increasing anddecreasing of the amount of gas in the gas-filled part of the system.

Disclosed in the publication is a pressure-measuring device with apressure transducer (38) which is sensitive to changes in a gas pressureand is connected via an air-filled line (37) to a housing (30). Thelatter is subdivided by a flexible membrane (33) into two chambers (31,32), the first chamber (31) being intended to have at least one openingfor feeding in liquids.

The second chamber (32) is intended to be connected to the line (37) ina gastight manner via a pipe connection and consequently capable ofbeing connected to the pressure transducer (38).

DE 29 30 869 C2 describes a pressure measuring capsule for fastening ona measuring transducer, which capsule has a housing in which there isformed within an annular bead a hollow space which is closed by themembrane adhesively attached onto the annular bead. It is intended thatthe hollow space can be filled with a liquid or a gas via two connectiontubes.

The main subject-matter of the description is the formation ofcontinuations of a cylindrical housing body of the pressure measuringcapsule to form a bayonet connection with correspondingly formedcounterparts on a measuring transducer. Emphasized in particular is anelastic design of the continuations to form defined snap-in endpositions of the mating parts of the bayonet connection, the pressuretransducer and pressure-measuring capsule having to be turned inrelation to one another to establish the mechanical connection. For theresilient design of the parts of the bayonet connection on the side ofthe pressure-measuring capsule, it is intended that productiontolerances can be compensated and a snap-in end position of the pressuremeasuring capsule in relation to the pressure transducer can beachieved.

The snap-in end position is at the same time intended to have the effectof exerting a defined prestressing force on the pressure transducers,the inventors envisaging that said position is to be consistentlyreproducible with different pressure transducers in such a way that zerobalancing of the evaluation electronics used is no longer required.

The cited publication describes a bayonet connection as particularlyadvantageous, the known disadvantages of which is [sic] in particular arubbing relative movement between the mating parts to be connected ofthe bayonet connection.

European Patent EP 0 330 891 B1 describes an arrangement fortransmitting the pressure of a fluid to another fluid. Proposed for thispurpose is an elongate housing, the inner space of which in the form ofan ellipsoid of revolution is divided into two spaces by a flexiblemembrane, it being possible for these two spaces to be arranged in sucha way that they form respectively neighboring areas of the inner space,or else concentrically. One of the spaces is intended to be providedwith an inlet opening and an outlet opening, to allow a first fluid toflow through it, such as blood for example.

The second space is provided with a single opening, via which a fluidwhich can be introduced into the second space is to be connected to anexternal pressure-measuring device, for example, in order for instanceto measure the pressure of the blood which is flowing through the firstspace.

It is described and claimed as essential for the invention to introducethe membrane into the housing in an unstretched or even folded state,the inventors concerned hoping that this brings about an improvement inthe pressure-measuring capabilities and in particular the measurementalso of negative pressures, without any further details of this beingspecified.

WO 97/39679 describes a coupling of a kind of pressure dome with atransducer, although the measuring chamber of the “pressure dome” is notclosed off from the surroundings by a membrane but by an isolating gel.When the pressure dome and transducer are being fitted, the flowabilityof the gel is intended to make it possible for air between the pressuredome and transducer to be pressed out through venting channels.

U.S. Pat. No. 4,562,845 describes a screw connection which leads to asealed coupling of a pressure dome to a transducer. Since the devicedescribed there is intended to be part of the system likewise describedthere for monitoring other transducers for a malfunction and forregistering air bubbles in blood-pressure monitoring systems, thepressure dome described there does not have a membrane for the sterilesealing of the fluid system from the surroundings and the transducer.

U.S. 4,462,409 discloses a pressure dome which, however, is not intendedfor looping into an extracorporeal circulation or for an infusionsolution to flow through but as a termination of a tap line which can behydraulically coupled via an infusion system to the circulation of apatient for pressure transmission. The measuring chamber of thispressure dome is separated from a transducer by means of a membrane. Thetransducer comprises a two-part housing, a first housing part (53 there)has a connecting area for the membrane of the pressure dome to beapplied. Provided on this connecting area of the housing part is a rib(63 a), which is intended to ensure a firm fit of a bead of the membranein a groove in the housing and consequently ensure reliable sealing ofthe measuring chamber.

However, the pressure dome is firmly connected to the housing part ofthe transducer by welding (loc. cit., column 4, lines 60-67), so that anarrangement with a disposable dome and a reusable transducer is notpossible.

U.S. Pat. No. 4,9200,972 [sic] describes a system comprising adisposable dome and a reusable transducer in which the measuring chamberin the dome and also the transducer are respectively closed off by meansof a membrane. Moreover, the teaching of this printed publication isconcerned with the replacement of oil for pressure transmission withinthe transducer by a gel, that [sic] only sets to the desired gel formafter it has been introduced in liquid form into the transducer housing,by heating up the transducer to 65° C. throughout for four hours. Thisarrangement is intended to increase the upper cut-off frequency of atransducer.

U.S. Pat. No. 5,551,300 discloses a set comprising a disposable pressuredome and a reusable transducer in which both the measuring chamber ofthe pressure dome and a liquid-filled measuring space of the transducerare closed off by a flat membrane adhesively attached to the respectivehousings. In this case, it is intended in respect of the transducer fora pressure equalization of the liquid-filled measuring space of thetransducer to be carried out in such a way that the measuring space isin flow connection with an equalization vessel, which is closed off fromthe surroundings by means of an elastic membrane.

The liquid system is in this case to be filled with a slight positivepressure, in order to ensure contact of the two pressure-transmittingmembranes and consequently the operational capability of the system. Toprevent a pressure equalization in the liquid system of the transducerduring the measurement, which is a prerequisite for pressuremeasurement, it is proposed to allow the transducer membrane to protrudeslightly and to provide a connection of the liquid-filled measuringchamber of the transducer to the pressure-equalizing vessel via a holeon the front side of the transducer, which is likewise covered by thetransducer membrane. By connection with the pressure dome, the membraneis mechanically applied to the housing of the transducer and, as aresult, the equalizing opening is closed, so that only the liquidremaining in the measuring chamber of the transducer can damp a signaltransmission to a piezo sensor.

DE 44 19 593 A1 discloses a device for measuring the pressure of amedium, in particular for pressure measurement in extracorporeal bloodcirculations, for example a dialysis system, in which a disposableelement which contains a measuring chamber and has two hose connectionsis provided, it being intended for the measuring chamber to be closed bya membrane which is to be placed in a peripheral groove and fastenedthere by means of a metallic clamping ring or by adhesive bonding. It isemphasized there as being particularly expedient that a peripheral beadis formed around the part of the measuring chamber which is inconnection with the membrane, in particular by an O-ring, which raisesthe membrane in the area of the connection to the measuring chamberabove the surface of the element. This is intended to make it possibleto obtain good coupling of the membrane to a pressure-measuringtransducer when the element is placed into a drawer of the measuringsystem and the pressure transducer is moved toward the element,preferably pneumatically or by means of a spindle.

However, in comparison with the prior-art pressure systems describedfurther above, it should be emphasized that the element does not ventitself and that numerous gaps and niches in which air bubbles can easilycollect during the filling of the system remain, in particular in thearea between the connecting opening to the measuring chamber and thesurrounding bead under the membrane. As already stated further above,not only is this disadvantageous with regard to the quality of thepressure transmission or the blood pressure measurement, but there isalso the considerable risk of clotting of the blood or the formation ofblood clots, which can, in particular in the case of extracorporealblood circulations, be life-threatening to the patient if such bloodclots are not intercepted before the blood is fed into the body.

This publication correspondingly does not contain any further details onthe measuring range of the system or the quality of the measurements, inparticular with respect to the cut-off frequency range, which wouldallow conclusions to be drawn concerning damping of the system by airbubbles remaining in the measuring chamber.

Furthermore, the device according to DE 44 19 593 A1 requires aconsiderable amount of effort for the measuring element to be fitted, inparticular with regard to the fitting of the membrane, whichadditionally leads to a great effort being required with regard tochecking how well it has been fitted for reasons of product liability.Furthermore, the large number of parts and, in particular, the hollowspaces formed between the membrane and the O-ring can also lead toproblems in sterilization. With regard to the effort required forfitting and quality assurance, this leads to such high costs that such ameasuring system is not acceptable for disposable use.

Furthermore, the corresponding arrangement of the pressure-measuringsensor requires great expenditure on apparatus, in particular withregard to the moving device for the sensor and the calibration of thesensor, dependent on the path of movement, so that such a system isunsuitable for everyday clinical use on account of the handling effortand enormous costs.

DT 21 29 670 A discloses what is known as a vacuum capsule, in which anelastic membrane is arranged in a fixed metal capsule, aspects describedas essential for the invention being the design of the membrane andfastening on a push rod, so that the membrane is applied to one side ofthe capsule during production and, after prestressing by a spring independence on the absolute air pressure to which the membrane issubjected on one side, can perform adjusting work via the push rod andthereby travel over a considerable fixed distance, in particular independence on the prestressing by means of the spring. However, thisdevice is not intended to allow throughflow and, on account of thedesign and intended purpose, is entirely unsuitable for measuringpressures, in particular in extracorporeal circulations.

DE 93 17 751 U1 discloses a pressure-indicating device that [sic] onreaching a certain predetermined pressure value indicates by a colorchange from green to red, or vice versa, visible through a transparentplate, and/or can actuate an electrical switch or button via a push rod.For this purpose, a liquid is enclosed in the intermediate space betweena membrane and a measuring housing. If the pressure on the other side ofthe membrane, which corresponds to the evaporation point of the liquid,falls below a limiting pressure, the liquid evaporates and the membraneabruptly travels together with a push rod fastened to it over a fixeddistance, so that a warning button or the like can be actuated by thepush rod. Appropriate coloring of the liquid obscures the view throughto a plate of a signalling color on the membrane, which suddenly becomesvisible when the liquid evaporates and is consequently intended toproduce a signalling effect.

It is quite evident that this device is not suitable for the continuousmeasurement of pressures.

Finally, WO 99/37983 discloses a pressure dome which is designed andsuitable in particular for use in extracorporeal blood circulations, forexample in hemodialysis, which furthermore can be handled particularlywell as a result of a releasable snap connection for fastening on atransducer.

SUMMARY OF THE INVENTION

The invention is therefore based on the object of providing a connectingelement of the type mentioned at the beginning which has improvedproperties with regard to its application-related reliability on the onehand and its handling on the other hand, and also makes it possible forpatients to be treated more safely and at lower cost.

This object is achieved according to the invention by a system elementof the type mentioned at the beginning in which the membrane has aperipheral bead which is located on the side of the membrane that facestoward the measuring chamber, the bead engaging in a groove which isformed in the housing and extends around the measuring chamber and thewall (inner wall) of the groove that faces toward the measuring chamberbeing lower than the wall (outer wall) of the groove that faces awayfrom the measuring chamber.

The object is also achieved according to the invention by a systemelement of the type mentioned at the beginning in which the measuringrange of the sensor covers at least differential pressures with respectto the surroundings of −250 mm Hg to +1000 mm Hg.

The object is finally achieved by a set of system elements according tothe invention.

The design according to the invention makes it possible for the firsttime to measure negative pressures in blood circulations to a greaterextent than can be produced by the pumping capacity of the heart alone.Furthermore, it is possible for the first time to measure directly inblood circulations negative pressures which are greater than that atwhich red blood corpuscles in human blood break up under prolongedexposure (hemolysis). Furthermore, it is possible for the first time toregister rates of pressure rise so high that adequately rapid automaticcontrol of peristaltic pumps connected in series is possible, so thatthe duration of the effect of a negative pressure on red bloodcorpuscles can be kept below a critical limit.

In medical technology, extracorporeal circulations are being used to agreatly increasing extent for the life-preserving function in criticalareas, such as open-heart surgery, multi-organ failure in cases ofinfections or accidents, detoxification by hemofiltration, for treatmentin the case of chronic renal insufficiency (hemodialysis) and also forobtaining blood preparations by cell separation. These extracorporealcirculations are operated by usually up to 4 pumps, in most casesperistaltic pumps, the pumps often being used in pairs in seriesconnection.

The pump heads of machines of extracorporeal circulations cannot servethe pump hose segments uniformly enough, so that positive and negativepressures occur in the hose system, in particular if the pump arrangeddownstream starts up quicker than the upstream pump. Under somecircumstances this produces a negative pressure for just fractions of asecond in the area of the line between the pumps. The constituents ofthe blood are very sensitive to negative pressures; the red bloodcorpuscles may break up and hemolysis occurs. To avoid critical pressureranges, nowadays T-pieces are looped in and pressure transducers on PCboards are used via tap lines and hydrophobicized filters with connectedair lines. This solution is so slow on account of the elastic buffervolumes that the critical pressure ranges are not avoided, at least fora short time, and consequently negative pressures down to −300 mm Hgnevertheless occur as pressure peaks in the range of a few seconds.

With the arrangement according to the invention, pressure changes in thenegative pressure range down to −400 mm Hg can be registered up to thecut-off frequency f_(G) of approximately 60 Hz. This is achieved, interalia, by it being possible for the membrane of the pressure dome to becoupled to the transducer membrane in such a way that it is sufficientlyfree of air and penetration of ambient air between the coupled membranesbeing sufficiently prevented by the design according to the invention,so that the pressure transmission to the transducer takes placesufficiently accurately even in the negative pressure range.

In a particularly preferred embodiment, the system element according tothe invention is characterized in that the difference in height of theouter wall with respect to the inner wall of the groove is less than theaverage thickness of the membrane in the area outside the peripheralbead. As a result, a particularly good sealing effect with thetransducer housing is achieved, in particular if the membrane protrudesat least approximately 0.1 mm, preferably at least approximately 0.3 mm,beyond the walls of the groove in the area of the bead after insertionof the bead into the groove.

By pressing the membrane against the inner wall of the dome, air bubblespossibly located in the area of the groove can no longer have adverseeffects by damping the signal or the negative pressure, nor can theycause clotting of the blood.

The membrane is expediently produced from an EPDM. It may, however, alsobe advantageous, in particular with regard to the sealing properties, ifthe membrane is produced from a TPE, in particular if the membrane isproduced from a TPE of the class SEBS.

For the use according to the invention, it is particularly expedient ifthe membrane seals a negative pressure of 530 hPa with respect to airwhen a pressing pressure of 60 N is applied to a finely turned steelsurface.

Falsifications of the measurement due to the stiffness of the membranecan be largely ignored if the membrane has within the bead a diameter ofat least 12 mm.

Particularly good fastening of the membrane can be obtained if the beadhas a width of approximately 2 mm in the longitudinal extent of themembrane.

A good compromise between transmission properties and productionexpenditure and also good mechanical durability of the membrane isobtained if the membrane has within the bead a thickness ofapproximately 0.4 mm to approximately 0.5 mm.

In an expedient embodiment of the invention, the measuring range of thesensor reaches at least down to differential pressures with respect tothe surroundings of −350 mm Hg; it is preferred for the measuring rangeof the sensor to cover at least differential pressures with respect tothe surroundings of −400 mm Hg to +3000 mm Hg.

The sealing integrity required for the measurement of negative pressurescan be obtained if the surrounding area is approximately technicallyplane and the surface of the surrounding area is smoothed to make itessentially free of scratches, preferably finely turned, ground orpolished, in particular if the surface of the surrounding area has anaveraged peak-to-valley height R_(Z) of no more than 20 μm (for examplein accordance with DIN 4768 Part 1) and/or the surface of thesurrounding area has a maximum peak-to-valley height R_(max) of no morethan 30 μm, preferably no more than approximately 20 μm.

When the pressure dome and transducer are put together, the inclusion ofeven extremely small air cushions can be largely avoided if thetransducer membrane is slightly pre-curved with respect to the areasurrounding it, in particular if the transducer membrane is pre-curvedwith respect to the area surrounding it by no more than 1 mm, preferablyno more than 0.6 mm.

Particularly good measuring results can be obtained if the transducermembrane is formed from an RTV silicone. It may also be advantageous forimproving the long-term stability, however, if the transducer membraneis formed from a polyurethane, preferably a TPE polyurethane.

The sealing integrity required for the measurement of negative pressurescan be obtained particularly reliably if the surrounding area has awidth of at least 2 mm.

In a preferred configuration, the set according to the invention ischaracterized in that at least one coupling device for the mechanicalconnection of the pressure dome and the transducer is provided and themembrane of the pressure dome is pressed in the area of the bead againstthe area surrounding the transducer membrane, so that an airtight sealtakes place between the membrane of the pressure dome and the transducerwhen the pressure dome and transducer are coupled, in particular if,when coupling has taken place, the membrane of the pressure dome ispressed in the area of the bead with at least 50 N, preferably by 90-110N, against the area surrounding the transducer membrane, and/or, oncecoupling has taken place, the membrane of the pressure dome is pressedtogether in the area of the bead by approximately 0.1 mm toapproximately 0.3 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is to be explained in more detail below on the basis ofexemplary embodiments represented in the drawings, in which:

FIGS. 1 a- 1 e show a pressure dome according to the invention invarious views;

FIG. 2 shows a transducer according to the invention in side view;

FIG. 3 shows a set according to the invention comprising a pressure domeand a transducer in a partially sectional view; and

FIG. 4 shows an enlarged detail of the representation according to FIG.3 as a sectional view.

The system element represented in FIGS. 1 a to 1 e for the releasablesealed connection of a transducer to a fluid system in the form of apressure dome 1 has two connections 2, 3 for the connection to aninfusion apparatus and to a patient, for example via a cemented-inthree-way cock, or for looping into an extracorporeal blood circulation,for example a dialysis apparatus, a heart-lung machine or a cellseparator. The connections 2, 3 respectively have a channel 4, 5,preferably with a conical packing seat, a cementing-in groove or someother sealed connection system 6 customary in medical technology. Thedimensioning of the connections 2, 3 may conform for example to DIN13090.

Connected to the inlet channel 4 and the outlet channel 5 via an inletopening 8 and an outlet opening 9 is a measuring chamber 7, so that aflow path from the inlet channel 4 through the measuring chamber 7 intothe outlet channel 5 is obtained.

The measuring chamber 7 is formed in a housing 10, which is produced asa one-piece injection-molded part, preferably from a transparentplastic, for example a polycarbonate. Part of the wall of the measuringchamber 7 is formed by an elastic membrane 11 (omitted in FIG. 1), forexample of an EPDM, a TPE, preferably of a TPE of the class SEBS, orsome other suitable material that is resistant to blood and/or infusionsolution and is physiologically harmless. Consequently, the connectingpart preferably comprises merely the membrane 11 and a one-pieceplastics injection-molded part.

The material of the membrane and its processing, for example injectionmolding, during production should create a membrane which seals anegative pressure of 530 hPa with respect to air when a pressingpressure of 60 N is applied to a finely turned steel surface.

At the height of the membrane 11 there is further provided a device forthe mechanical coupling of the connecting element to the transducer (12)which is part of a releasable spreading-in connection, which is formedby claw-shaped retaining elements in the form of hooks 13 for engagementin a corresponding groove or undercut 14 of the transducer 12 or anassociated fastening device.

The hooks 12 are formed by resilient continuations of the housing 10 andpreferably six of them are evenly distributed over the circumference ofthe pressure dome 1. In this arrangement, the resilient connection ofhooks 12 to the housing 10 is designed such that a pre-stressing forceof at least approximately 60 N can be maintained.

As can easily be seen in FIG. 3, the pressure dome 1 can be placed ontothe transducer 12 by simply pressing on the transducer 12 in a directionapproximately perpendicular to a plane which is defined by the membrane11.

The measuring chamber 7 is bounded radially outward by part of its wallforming an edge 15, the inlet opening 8 and outlet opening 9 buttingagainst this edge 15. In between and opposite the membrane 11, a ceiling(not represented) of the measuring chamber 7 is formed by part of thewall.

Near the membrane 11, the edge 15 is formed by an inner wall 16, whichtogether with an approximately 0.5 mm higher wall (outer wall 17) thatfaces away from the measuring chamber and a base 18 forms a groove 19which extends around the measuring chamber 7.

For fastening the membrane 11, a bead 20 which runs around the edge ofthe membrane 11 and is located on the side of the membrane 11 that facestoward the measuring chamber 7 is pressed into the groove 19. The bead20 of the membrane 11 may have a width of approximately 2 mm, measuredin the longitudinal extent of the membrane 11. The planar area of themembrane 11 within the bead edge 20 expediently has a diameter of atleast 12 mm, with a suitable thickness in this area of approximately 0.4mm to approximately 0.5 mm.

The dimensioning of the inner wall 16 in relation to the depth of thegroove 19 is in this case performed in such a way that, after pressingthe bead 20 into the groove 19, the flat area 21 of the membrane liftsup from the inner wall 16 by a minimal gap. When the pressure dome 1 iscoupled to the transducer 12, the gap is closed without the membrane 11deforming to any appreciable extent, thereby preventing the membrane 11from arching up.

The difference in height of the outer wall 17 with respect to the innerwall 16 is less than the average thickness of the flat area 21 of themembrane. The height of the outer wall 17 is in this case expedientlyfixed in such a way that the membrane 11 protrudes at leastapproximately 0.1 mm, preferably at least approximately 0.3 mm, beyondthe outer wall 17 of the groove 19 in the area of the bead 20 afterinsertion of the bead 20 into the groove 19. When the pressure dome 1 isplaced onto the transducer 12, the membrane 11 is slightly squeezed inthe area of the bead 20, whereby a kind of sealing lip 25 against airpenetrating from the outside is formed (FIG. 4) in the area of the gap22 which is produced between the outer wall 17 and the correspondingabutment on the transducer 12, the area 24 surrounding the transducermembrane 23.

For use of the connecting element according to the invention inblood-carrying systems, for example in dialysis, the inlet channeland/or the outlet channel 5 is arranged inclined with respect to a planeparallel to the membrane 11. It is advantageous in this case if theinclination of the inlet channel 3 and/or of the outlet channel 5 withrespect to a plane parallel to the membrane 11 is approximately 15° to45°, preferably 15° to 30°, particularly preferably approximately 20°.As a result, the forces on cells present in the fluid when it flowsthrough the connecting element can be minimized. In this way, the riskof a hemolysis, that is breaking up of the red blood corpuscles, can beavoided to the greatest extent by the flow in the pressure dome alone.

The system element represented in FIG. 2 for the releasable sealedconnection of a transducer to a fluid system in the form of a transducer12 contains at least one sensor (not represented) for convertingpressures and pressure changes into electrical signals, the sensor beingarranged in a housing 26 and at least part of the housing 26 beingfilled with a fluid or gel which transmits pressure, and the housing 26being closed by a transducer membrane 23 on one side, the housing 26being further configured in the area 24 surrounding the transducermembrane 23 for applying a pressure dome 1, described further above, thesurrounding area 24 essentially encompassing the transducer membrane 23in a ring shape, and the measuring range of the sensor covering at leastdifferential pressures with respect to the surroundings of −250 mm Hg to+1000 mm Hg, as far as possible down to differential pressures withrespect to the surroundings of −350 mm Hg. It is preferred for themeasuring range of the sensor to cover at least differential pressureswith respect to the surroundings of −400 mm Hg to +3000 mm Hg.

The surrounding area 24 is approximately technically plane and thesurface of the surrounding area 24 is smoothed to make it essentiallyfree of scratches. The surface of the surrounding area 24 should have anaveraged peak-to-valley height R_(Z) of no more than 20 μm, (for examplein accordance with DIN 4768 Part 1) and/or a maximum peak-to-valleyheight R_(max) of no more than 30 μm, preferably no more thanapproximately 20 μm.

The sealing effect required for the measurement of negative pressurescan be obtained particularly reliably if the surrounding area 24 has awidth of at least 2 mm.

When the pressure dome 1 and transducer 12 are put together (FIG. 3),the inclusion of even extremely small air cushions can be largelyavoided, since the transducer membrane 23 is slightly pre-curved withrespect to the area 24 surrounding it, for example by no more than 1 mm,preferably no more than 0.6 mm.

The transducer membrane 23 may be formed from an RTV silicone. It mayalso be advantageous for improving the long-term stability, however, ifthe transducer membrane 23 is formed from a polyurethane, preferably aTPE polyurethane. A coupling device for the mechanical connection of thepressure dome 1 and the transducer 12 may be formed by the hooks 13 anda corresponding undercut 14 on the transducer 12. However, other knowncoupling mechanisms also come into consideration, although a rotatingmovement between the membrane 11 and transducer membrane 23 should beavoided. For this purpose, it is possible for example for the hooks andundercut to be changed over. A different snap connection may also beprovided, for example as proposed in WO 99/37983. Furthermore, bayonetconnections, preferably with a screw collar ring, come intoconsideration, also correspondingly as a screw connection if excessivetightening of the connection is prevented by suitable spacers or thelike. Finally, pivotably mounted clamps or toggle catches similar toclosing hooks may also be provided.

When coupling has taken place (see FIG. 4), the membrane 11 of thepressure dome 1 is pressed in the area of the bead 20 against the area24 surrounding the transducer membrane 23, so that an airtight sealtakes place between the membrane 11 of the pressure dome 1 and thetransducer 12; when coupling has taken place, the membrane 11 of thepressure dome 1 is pressed in the area of the bead 20 with at least 50N, preferably by 90-110 N, against the area 24 surrounding thetransducer membrane 23, and/or, once coupling has taken place, themembrane 11 of the pressure dome 1 is pressed together in the area ofthe bead 20 by approximately 0.1 mm to approximately 0.3 mm.

1. A system element for the releasable sealed connection of a transducerto a fluid system, with a measuring chamber (7) which can be connectedto the fluid system in such a way as to allow throughflow, the measuringchamber (7) being formed in a housing (10) and part of the wall of themeasuring chamber (7) being formed by a membrane (11) which isconsiderably more flexible than the rest of the wall (15) of themeasuring chamber (7), characterized in that the membrane (11) has aperipheral bead (20) which is located on the side of the membrane (11)that faces toward the measuring chamber (7), the bead (20) engaging in agroove (19) which is formed in the housing (10) and extends around themeasuring chamber (7) and the wall (inner wall 16) of the groove (19)that faces toward the measuring chamber (7) being lower than the wall(outer wall 17) of the groove (19) that faces away from the measuringchamber (7).
 2. The system element as claimed in claim 1, characterizedin that the difference in height of the outer wall (17) with respect tothe inner wall (16) of the groove (19) is less than the averagethickness of the membrane (11) in the area (21) outside the peripheralbead (20).
 3. The system element as claimed in claim 1, characterized inthat the membrane (11) protrudes at least approximately 0.1 mm,preferably at least approximately 0.3 mm, beyond the walls (16, 17) ofthe groove (19) in the area of the bead (20) after insertion of the bead(20) into the groove (19).
 4. The system element as claimed in claim 1,characterized in that the membrane (11) is produced from an EPDM.
 5. Thesystem element as claimed in claim 1, characterized in that the membrane(11) is produced from a TPE.
 6. The system element as claimed in claim5, characterized in that the membrane (11) is produced from a TPE of theclass SEBS.
 7. The system element as claimed in claim 1, characterizedin that the membrane (11) seals a negative pressure of 530 hPa withrespect to air when a pressing pressure of 60 N is applied to a finelyturned steel surface.
 8. The system element as claimed in claim 1,characterized in that the membrane (11) has within the bead (20) adiameter of less than 12 mm.
 9. The system element as claimed in claim1, characterized in that the bead (20) has a width of approximately 2 mmin the longitudinal extent of the membrane (11).
 10. The system elementas claimed in claim 1, characterized in that the membrane (11) haswithin the bead (20) a thickness of approximately 0.4 mm toapproximately 0.5 mm.
 11. A system element for the releasable sealedconnection of a transducer (12) to a fluid system, containing at leastone sensor for converting pressures and pressure changes into electricalsignals, the sensor being arranged in a housing (26) and at least partof the housing (26) being filled with a fluid or gel which transmitspressure, and the housing (26) being closed by a transducer membrane(23) on one side, the housing (26) being further configured in the area(24) surrounding the transducer membrane (23) for applying a systemelement as claimed in one of the preceding claims, the surrounding area(24) essentially encompassing the transducer membrane (23) in a ringshape, characterized in that the measuring range of the sensor covers atleast differential pressures with respect to the surroundings of −250 mmHg to +1000 mm Hg.
 12. The system element as claimed in claim 11,characterized in that the measuring range of the sensor reaches at leastdown to differential pressures with respect to the surroundings of −350mm Hg.
 13. The system element as claimed in claim 12, characterized inthat the measuring range of the sensor covers at least differentialpressures with respect to the surroundings of −400 mm Hg to +3000 mm Hg.14. The system element as claimed in claim 11, characterized in that thesurrounding area (24) is approximately technically plane and the surfaceof the surrounding area (24) is smoothed to make it essentially free ofscratches, preferably finely turned, ground or polished.
 15. The systemelement as claimed in claim 11, characterized in that the surface of thesurrounding area (24) has an averaged peak-to-valley height R_(Z) of nomore than 20 μm.
 16. The system element as claimed in claim 11,characterized in that the surface of the surrounding area (24) has amaximum peak-to-valley height R_(max) of no more than 30 μm, preferablyno more than approximately 20 μm.
 17. The system element as claimed inclaim 11, characterized in that the transducer membrane (23) is slightlypre-curved with respect to the area (24) surrounding it.
 18. The systemelement as claimed in claim 17, characterized in that the transducermembrane (23) is pre-curved with respect to the area surrounding it byno more than 1 mm, preferably no more than 0.6 mm.
 19. The systemelement as claimed in claim 11, characterized in that the transducermembrane (23) is formed from an RTV silicone.
 20. The system element asclaimed in claim 11, characterized in that the transducer membrane (23)is formed from a polyurethane, preferably a TPE polyurethane.
 21. Thesystem element as claimed in claim 11, characterized in that thesurrounding area (24) has a width of at least 2 mm.
 22. A set comprisinga system element as claimed in claim 1, characterized in that the systemelement is designed as a pressure dome (1).
 23. The set as claimed inclaim 22, characterized in that at least one coupling device (13, 14)for the mechanical connection of the pressure dome (1) and thetransducer (12) is provided and the membrane (11) of the pressure dome(1) is pressed in the area of the bead (20) against the area (24)surrounding the transducer membrane (23), so that an airtight seal takesplace between the membrane (11) of the pressure dome (1) and thetransducer (12) when the pressure dome (1) and transducer (12) arecoupled.
 24. The set as claimed in claim 22, characterized in that, whencoupling has taken place, the membrane (11) of the pressure dome (1) ispressed in the area of the bead (20) with at least 50 N, preferably by90-110 N, against the area (24) surrounding the transducer membrane(23).
 25. The set claimed in one of claims 22, characterized in that,once coupling has taken place, the membrane (11) of the pressure dome(1) is pressed together in the area of the bead (20) by approximately0.1 mm to approximately 0.3 mm.